CODEWITHSUNDEEP
androidmultithreadingconfigurationandroid-activityorientation
Ted
Ted

Reputation: 43

Activity's background thread & configuration change

I have seen some discussion here on Stack Overflow related to using Activity.onRetainNonConfigurationInstance() to maintain a background thread started by one instance of an Activity and pass it to the next instance of the Activity which results, for example, when the phone's orientation changes from portrait to landscape.

The discussions do not specify exactly what can be done with the thread wrapped in the Object returned from onRetainNonConfigurationInstance().

For example: 1. Is there a way to actually keep the background thread running using this technique? 2. Do you need to somehow pause the thread when the previous instance of Activity is going away and then restart it again in the new instance?

Can anyone provide a short example?

Any details would be appreciated.

Upvotes: 2

Views: 1763

Answers (2)

Jon O
Jon O

Reputation: 6591

The thread will continue to run. For an example of what you can do with this, you can check out the android Ignition project and its IgnitedAsyncTask (and related examples).

The idea is that you will maintain a reference to your thread (usually an AsyncTask) somewhere in your Activity, and occasionally your thread (again, especially if it's an AsyncTask) will require a reference to a Context in order to perform some kind of UI update upon the conclusion of its background task. You will need to make sure that the Context (and anything derived from it--like a TextView or the like) to which your thread has a reference is non-null, or else it will crash.

You might use getLastNonConfigurationInstance() to set your Activity's reference to the thread, and then call a setter on the thread to set its Context reference (to avoid any related null pointer crash).

Upvotes: 0

David Wasser
David Wasser

Reputation: 95656

You can return anything you want to onRetainNonConfigurationInstance(). If you have a Thread that you want passed from one instance of the Activity to another, you can either return it directly, or put it inside another object that you return from onRetainNonConfigurationInstance(). You don't need to pause the thread or interact with it in any way. It just keeps running as if nothing happened.

The only thing you need to be concerned about is how the Thread interacts with the Activity (if at all). If the thread will call the Activity back (to indicate progress or something like that) then you somehow need to give the thread a reference to the new Activity, as the old Activity will be dead.

What do you want to do in your background thread?

EDIT (add more details about threads/activities):

Threads have their own lifetimes which are completely disconnected from Activities. If you create a Thread in an Activity and start it, it will run to completion no matter what your Activity does. The only thing that will stop the thread explicitly is if Android decides to kill your process (which it may do if your process contains no active activities).

Upvotes: 2

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